Current panel sawing processes by which large panels are cut into smaller sub-panels used, for example, in the furniture industry, involve the use of apparatus which, starting from a vertical pile of panels or sheets positioned on an elevator table: select a certain number of panels from the top of the pile; grip the selected panels to form a stack; and transfer the stacks thus formed at regular intervals to a sawing station.
In an apparatus of this kind made by the same Applicant as this invention, panel selection is achieved by the combined operation of a stop element that references the top of the pile and a panel selection element. Both these elements are mounted on a crossbar that moves to and from above the pile of panels in a direction parallel to itself between a position in which it receives the pile of panels and a position in which it feeds the panels to the sawing station.
The selection element operates in a development plane below the reference stop at the top of the pile at a distance corresponding to the thickness of the required stack of panels.
A side wall abuts against one side of the pile at the face opposite the face of it nearest the selection element. Opposing the reaction exerted by the side wall and in some cases also acting in conjunction with the simultaneous lowering of the elevator table, the selection element is actuated in such a way as to create a gap in the pile of panels separating a certain number of stacked panels en bloc from the rest of the panels in the pile below that will form the subsequent stacks.
The crossbar also mounts gripper clamps having parallel, horizontal jaws. The lower jaw of each of the clamps penetrates the gap created and kept open by the selection element and, working in conjunction with the upper jaw that acts on the panel at the top, clamps the stack of panels separated en bloc from the rest of the pile. The crossbar is then moved horizontally with the jaws in the clamped position in such a way as to carry the stack of panels thus selected towards the sawing station.
In the apparatus described above, the clamps are fixed to the crossbar and other than the limited forward movement of the jaws required to enable them to grip the edge of the stack, and the movements required for positional adjustment, the clamps are in practice unable to move relative to the crossbar.
The apparatus has proved highly satisfactory when working with relatively thick panels, that is to say, panels whose flexural rigidity is sufficiently high to prevent them from sagging and enabling them to remain in a substantially flat, straight condition.
However, when the panels to be selected and moved are thin, that is to say, when the surface area of the panels is very large compared to their thickness, the apparatus described above has some significant disadvantages.
Thus, once the selection element has selected the panels required to form a stack and lifted them off the rest of the pile, the parts of the panels at the sides of the selection element may bend under their own weight on account of their flexibility, making it difficult for the clamps to grip them securely or, in some cases, causing the two clamps to grip a different number of panels.
When the crossbar starts moving, the panel (or panels) gripped on one side only tends to move out of alignment with respect to the rest of the stack, causing moving parts to jam. When this happens, the machine has to be brought to an immediate stop and the panels or, even worse, the machine itself, may be seriously damaged.
The flexibility of the panels may also lead to a problem of another kind, due to the fact that one or two panels at the top of the pile below the gap created by the selection element may be accidentally dragged along with the selected stack.
Thus, if the panels are very flexible, the parts of the selected stack furthest away from the clamps touch the panel at the top of the pile below and, when the crossbar starts moving towards the sawing station, the bottom of the selected stack scrapes against the top of the pile, with the risk of dragging the top panels along with it.
If the stack contains a large number of panels, that is to say, if it is very heavy, the weight of the stack bears down on the rest of the panels in the pile below and the resulting friction between the bottom of the stack being moved and the panel at the top of the remaining pile may be strong enough to move the top panel or panels out of alignment relative to the rest of the pile.
In this case, too, the machine has to be stopped, the damaged panels removed and normal operation resumed, with all the obvious disadvantages that this involves, especially when the machine forms part of a more complex production line.